In radio systems, in particular in the mobile radio sector, a common antenna is frequently used for transmitted and received signals. In this context, the transmitted and received signals each use different frequency ranges, and the antenna has to be suitable for transmitting and receiving in the two frequency ranges. Therefore, to separate the transmitted and received signals, a suitable frequency filtering is required, by means of which, on the one hand, the transmitted signals are passed from the transmitter to the antenna and, on the other hand, the received signals are passed from the antenna to the receiver. In order to separate the transmitted and received signals or to combine or separate mobile radio bands, nowadays inter alia high frequency filters in coaxial construction are used. In this case, two interconnected high frequency filters form what is known as a duplex separating filter, which allows transmitters and receivers to be interconnected in a largely decoupled manner on a common antenna. For example, a pair of high frequency filters can be used, which both allow a certain frequency band to pass (bandpass filters). As an alternative, a pair of high frequency filters can be used, which both block a certain frequency band (band-stop filters). Furthermore, a pair of high frequency filters can be used, of which one filter allows frequencies below a frequency between the transmitting and receiving band to pass and blocks frequencies above this frequency (low-pass filters), and the other filter blocks frequencies below a frequency between the transmitting and receiving band and allows frequencies above this to pass (high-pass filters). Further combinations of the aforementioned filter types are also conceivable. This also applies for high frequency filters that are formed as single filters, i.e. have just one input and one output.
High frequency filters that consist of coaxial resonators can be easily produced from milled or cast parts. Furthermore, these resonators ensure a high electrical quality and relatively high thermal stability.
However, in order to be able to achieve optimal filtering results, fine-tuning is required after production.
WO 2014/063829 A1 discloses a high frequency filter in which tuning elements can be inserted into the resonator. Inserting said tuning elements causes a change in the resonance frequency of the high frequency filter. The tuning elements are screwed in from outside the high frequency filter. This occurs via a threaded connection between the tuning element and a bush that is inserted into an opening of the high frequency filter. WO 2014/063829 A1 is disadvantageous in that introducing a bush of this kind together with the necessary thread is complex and in that automated tuning can be achieved only with difficulty.
DE 26 20 769 A1 discloses a tunable high frequency filter. An internal conductor is galvanically connected to a front face of the high frequency housing and extends from this front face towards an opposing further front face of the high frequency housing. A pin is arranged on this further front face and extends towards the internal conductor. The internal conductor comprises an internal conductor bore, into which the pin protrudes.
The internal conductor is formed in two parts, the second part being longitudinally movable in a telescopic manner in the first part, meaning that the internal conductor is formed in multiple parts and is adjustable in length. The distance by which the second part of the internal conductor can be moved relative to the first part of the internal conductor can be set by a threaded rod that is rigidly connected to the second part of the internal conductor and can be actuated from the outside of the high frequency filter.
DE 10 2010 056 048 A1 discloses a further high frequency filter. Said filter comprises an internal conductor that is galvanically connected to a housing base of the high frequency filter and extends from the housing base towards the housing cover. At the same time, a first tuning element extends from the housing base towards the housing cover. The internal conductor comprises an internal conductor bore, into which the first tuning element extends. A second tuning element can be screwed or pushed into the internal conductor bore from outside the high frequency filter, by means of a thread.
A disadvantage of DE 10 2010 056 048 A1 is that abrasion can result when using a thread, which abrasion leads to intermodulation products. In the event that the second tuning element is merely pushed in, the diameter of the internal conductor bore and the outside diameter of the tuning element must be very precisely matched to one another in order to ensure permanent retention.
The object of the present invention is therefore that of providing a high frequency filter and a method for tuning the high frequency filter, which filter and method are more cost-effective and simpler to produce and carry out, respectively, and provide better results over a longer period of time compared with the prior art.
The high frequency filter in coaxial construction according to the invention comprises at least one resonator comprising a first internal conductor and comprising an external conductor housing. The external conductor housing comprises a housing base, a housing cover that is spaced apart from the housing base, and a peripheral housing wall between the housing base and the housing cover. A first internal conductor is galvanically connected to the housing base and extends in the axial direction from the housing base towards the housing cover. The first internal conductor ends at a distance from the housing cover and/or is galvanically isolated from the housing cover. The resonator further comprises a second internal conductor that is galvanically connected to the housing cover and extends in the axial direction from the housing cover towards the housing base. The first and the second internal conductors are axially immovable, i.e. are not adjustable in length, and are mutually coaxial. The first internal conductor and the housing base, just like the second internal conductor and the housing cover, are preferably formed integrally. The first and/or second internal conductor comprise an internal conductor bore. The internal conductor bore of the first or second internal conductor penetrates the external conductor housing and leads into an insertion opening.
A tuning element is arranged inside the internal conductor bore of the first or second internal conductor, so as to be axially movable. In this case, the tuning element is designed and/or arranged such that a portion of the tuning element enters the free clearance between the two internal conductors to varying extents. Furthermore, a bush or a sleeve is arranged in a form-locked or force-locked manner inside the internal conductor bore, between the first internal conductor and the tuning element or the second internal conductor and the tuning element. Alternatively or additionally thereto, the tuning element comprises a region having a widened diameter, this region being located either in the centre of the tuning element and/or at the end of the tuning element that is closer to the insertion opening. The region having the widened diameter is elastically deformable, at least in the radial direction, towards the longitudinal axis that extends centrally through the tuning element.
It is particularly advantageous for the tuning element to be axially movable, meaning that no thread is required. Since the tuning element can be moved axially in the internal conductor bore in a threadless manner, smaller filters can be produced because the diameter of the internal conductor bore is no longer restricted to a minimum diameter that was previously necessary in order to still be able to accommodate a thread. Omitting the thread also results in less metal abrasion during tuning, which abrasion would cause interfering effects in the high frequency filter (PIM—passive intermodulation). The tuning element can be pressed into the internal conductor bore, for example, preferably injected therein by means of compressed air. It is further advantageous for there to be a further second internal conductor in addition to a first internal conductor, the two internal conductors extending coaxially towards one another. Improved filter effects can be achieved as a result, it being possible for the high frequency filter to be tuned particularly easily by means of the tuning element being inserted into the resonator to varying extents. Using a bush or a sleeve means that the internal conductor bore does not need to be additionally specially further processed in order to ensure that the tuning element is fitted optimally or exactly. Furthermore, the internal conductor bore can be produced having a consistent diameter. Subsequently, the diameter of the tuning element can then be selected as desired, by selecting the appropriate bush or sleeve. Since the tuning element comprises a resilient region having a widened diameter, it is possible to ensure, without using a thread, that the tuning element is fitted securely and permanently inside the internal conductor bore.
Threadless movement is also not known in this context. Although U.S. Pat. No. 4,460,878 discloses threadless movement of different components, this does not relate to a tuning element but instead to an extension of the internal conductor. Said document does not disclose the use of a plurality of internal conductors and the immovable attachment thereof to a housing cover and to a housing base, or the fact that the internal conductor bore of the first or second internal conductor opens into an insertion opening on the external conductor housing and is thus directly accessible from the outside. Moreover, the use of a bush or sleeve is not disclosed. It is also not disclosed that the tuning element is intended to comprise a widened region that is resilient.
The method according to the invention for tuning the high frequency filter, as has been described for example according to independent claim 1, comprises various method steps. In a first method step, the high frequency filter is sealed. In the further method step, a connection is produced between an attachment device arranged on the tuning element and a coupling means of the adjusting device. In a following method step, the tuning element is inserted into the internal conductor bore of the first or second internal conductor. These steps can be carried out in any desired sequence. The filter properties are subsequently measured, the tuning element being pushed further towards the insertion opening or away from the insertion opening, using the coupling means of the adjusting device, depending on the measurement result. Subsequently, the method steps of “measuring” and “moving” are repeated until the high frequency filter has the desired filter properties. When this state has been reached, an adhesive connection is added between the tuning element and the internal conductor bore of the first or second internal conductor, as a result of which connection the tuning element is fixed, in a permanent and immovable manner, in the axial position thereof inside the internal conductor bore.
A particular advantage in this case is the axial movement of the tuning element inside the internal conductor bore, which movement can be achieved in a particularly simple manner using the coupling means, which is a component of the adjusting device, by means of a linear motor or stepper motor.
In addition, widening a region of the tuning element means that said region having the widened diameter is oversized relative to the internal conductor bore, and the remaining region is undersized relative to the internal conductor bore. As a result of the region having the widened diameter, the region without the widened diameter is also arranged centrally inside the internal conductor bore. In this case, the tuning element rests inside the internal conductor bore in a force-locked manner, but can nonetheless be moved by means of a stepper motor or a linear motor. The tuning element no longer moves autonomously, and therefore said element can, for example, very easily be permanently fixed, by means of an adhesive connection, to the internal conductor bore, i.e. to the inside wall of the internal conductor bore.
In order to facilitate the elastic deformability, the region having the increased diameter can be slotted at least in part. As a result, the tuning element can be inserted more easily into the internal conductor bore, it nonetheless being ensured, at the same time, that the tuning element rests inside the internal conductor bore in a force-locked manner and that the position of said element does not change on account of gravity alone, or as a result of shocks during the process for producing and/or tuning the high frequency filter.
The tuning element is arranged in the internal conductor bore of the first internal conductor and protrudes therefrom into the internal conductor bore of the second internal conductor, the front faces of the two internal conductors preferably not touching, and further preferably being arranged so as not to have any mutual overlap, such that neither of the internal conductors enters the other internal conductor in each case. It would also be possible for the tuning element to be arranged in the internal conductor bore of the second internal conductor and to protrude therefrom into the internal conductor bore of the first internal conductor. Here, too, the two internal conductors should not touch and can in addition be arranged so as not to have any mutual overlap. Overlapping would, however, also be possible. In another embodiment, the internal conductor bore of the first internal conductor has a larger diameter, overall, than the second internal conductor, the second internal conductor then entering the internal conductor bore of the first internal conductor at least in part. A clearance is formed between the two internal conductors, which in this case overlap at least in part radially towards the outside, i.e. the internal conductors do not touch. In this case, the tuning element is designed and/or arranged such that at least a portion of the tuning element enters the free clearance between the two internal conductors to varying extents. In this case, the tuning element can be mushroom-shaped for example. The same applies for the case in which the internal conductor bore of the second internal conductor has a larger diameter than the first internal conductor, and said first internal conductor enters the internal conductor bore of the second internal conductor.
In a further embodiment, it may also be possible for the end of the tuning element furthest from the insertion opening to comprise a receiving opening. In this case, the second internal conductor can enter the receiving opening of the tuning element when the tuning element is arranged in the internal conductor bore of the first internal conductor. The same applies when the tuning element is arranged in the internal conductor bore of the second internal conductor, the first internal conductor entering said second internal conductor in this case.
The corresponding embodiments regarding the arrangement of the tuning element relative to the first and/or second internal conductor, and the arrangement of the two internal conductors relative to one another, are dependent on the frequency range over which the high frequency filter must be tuned.
The internal conductor bore preferably widens towards the insertion opening, i.e. towards the outside of the external conductor housing. This widening can be tapered or conical in longitudinal section, for example. A parabolic widening is also possible. Not only is the insertion of the tuning element facilitated as a result, but said widening can also make it easier to receive adhesives, by means of which the tuning element can be fixed in the internal conductor bore in a permanent and rigid manner.
It is also possible for the tuning element to comprise a first sliding surface as a peripheral surface, which surface extends at least in the region in which the tuning element is guided inside the internal conductor bore. A second sliding surface is preferably located in the internal conductor bore as an inside wall, it being necessary for the coefficients of friction of the first and the second sliding surfaces to be selected such that the tuning element is arranged securely inside the internal conductor bore and can be moved axially after insertion only by using a stepper motor or a linear motor.
The bush or sleeve is preferably resilient, and preferably further consists of a dielectric material. The bush is used to produce a force-locked connection to the tuning element. The bush can consist of a rubber compound for example. The bush is arranged in a form-locked or force-locked manner inside the internal conductor bore of the first or second internal conductor. Instead of a bush, as mentioned, a sleeve can also be used, the sleeve being pulled over the tuning element before the tuning element is inserted into the internal conductor bore. In contrast, a bush is already located inside the internal conductor bore before the tuning element is inserted. Both the bush and the sleeve, which preferably both consist of a dielectric material, furthermore allow the tuning element to also be formed of an electrically conductive material instead of a dielectric material, of which said element is preferably formed.
When a bush is used, the ends thereof preferably comprise an at least partially peripheral flange, so that the bush is arranged in an axially immovable manner inside the internal conductor bore of the first or second internal conductor. The at least partially peripheral flange of a first end of the bush is supported on a shoulder arranged inside the internal conductor bore of the first or second internal conductor. The internal conductor bore therefore comprises a step and is therefore tapered at least in part. The likewise at least partially peripheral flange of a second end of the bush is supported on the insertion opening of the internal conductor bore, on an outer side of the external conductor housing.
The end of the tuning element that is closer to the insertion opening additionally comprises an attachment device. Said attachment device makes it possible to connect an auxiliary device to the tuning element, it being possible for a tensile and/or compressive movement to be transmitted via said auxiliary device to the tuning means, as a result of which said tuning means can be moved back and forth inside the internal conductor bore. Said auxiliary device is preferably an adjusting device that comprises a coupling means, the coupling means being connected to the attachment device. At least part of the coupling means can be or is inserted into the insertion opening from outside said opening. The mentioned tensile and compressive forces can be transmitted via said connection between the attachment device and the coupling means. In this case, the adjusting device in addition also comprises the linear or stepper motor.
The connection between the attachment device and the coupling means is formed as a detachable connection. In particular a bayonet connection or a screw connection or a latching mechanism or a vacuum connection are possible for this purpose.
In order to be able to ensure frictionless movement of the tuning element inside the internal conductor bore, the attachment device and the tuning element are preferably formed integrally.
After the filter has been tuned, the tuning element is preferably permanently fixed inside the internal conductor bore. This is achieved by means of an adhesive connection, the adhesive connection being introduced into the internal conductor bore from outside the external conductor housing, via the insertion opening, as a result of which the end of the tuning element that is closer to the insertion opening is connected to the inside wall of the internal conductor bore.